There are a variety of known ways to print hard copy in black and white and in color. Traditional techniques include letterpress printing, rotogravure printing and offset printing. These conventional printing processes produce high quality copies. However, when only a limited number of copies are required, the copies are relatively expensive. In the case of letterpress and gravure printing, the major expense results from the fact that the image is cut or etched into the plate using expensive photographic masking and chemical etching techniques. Plates are also required in offset lithography. However, the plates are in the form of mats or films that are relatively inexpensive to make. The image is present on the plate or mat as hydrophilic (water-receptive) and hydrophobic (water-repellent) surface areas; hydrophobic areas are generally oleophilic, or ink-receptive, as well. In wet lithography, water and then ink are applied to the surface of the plate. Water tends to adhere to the hydrophilic or water-receptive areas of the plate, creating a thin film of water thereon which does not accept ink. The ink does adhere to the hydrophobic areas of the plate and those inked areas, usually corresponding to the printed areas of the original document, are transferred to a relatively soft blanket cylinder and, from there, to the paper or other recording medium brought into contact with the surface of the blanket cylinder by an impression cylinder.
Most conventional offset plates are also produced photographically. In a typical negative-working, subtractive process, the original document is photographed to produce a photographic negative. The negative is placed on an aluminum plate having a water-receptive oxide surface that is coated with a photopolymer. Upon being exposed to light through the negative, the areas of the coating that received light (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process which removes the noncured areas of the coating that did not receive light (corresponding to the light or background areas of the original). The resultant plate now carries a positive or direct image of the original document.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required, each of which is usually made photographically as aforesaid. In addition to preparing the appropriate plates for the different colors, the plates must be mounted properly on the plate cylinders in the press and the angular positions of the cylinders coordinated so that the color components printed by the different cylinders will be in register on the printed copies.
The development of lasers has simplified the production of lithographic plates to some extent. Instead of applying the original image photographically to the photoresist-coated printing plate as above, an original document or picture is scanned line-by-line by an optical scanner which develops strings of picture signals, one for each color. These signals are then used to control a laser plotter that writes on and thus exposes the photoresist coating on the lithographic plate to cure the coating in those areas which receive light. That plate is then developed in the usual way by removing the unexposed areas of the coating to create a direct image on the plate for that color. Thus, it is still necessary to chemically etch each plate in order to create an image on that plate.
There have been some attempts to use more powerful lasers to write images on lithographic plates by volatilizing the surface coating so as to avoid the need for subsequent developing. However, the use of such lasers for this purpose has not been entirely satisfactory because the coating on the plate must be compatible with the particular laser; this requirement limits the choice of coating materials. Also, the pulsing frequencies of some lasers used for this purpose are so low that the time required to produce a halftone image on the plate is unacceptably long.
There have also been some attempts to use scanning E-beam apparatus to etch away the surface coatings on plates used for printing. However, such machines are very expensive. In addition, they require that the workpiece, i.e. the plate, be maintained in a complete vacuum, making such apparatus impractical for day-to-day use in a printing facility.
Images have also been applied to lithographic plates by electroerosion. A type of plate suitable for imaging in this fashion, and disclosed in U.S. Pat. No. 4,596,733, has an oleophilic plastic substrate, e.g. Mylar brand plastic film, having a thin coating of aluminum metal with an overcoating that contains conductive graphite; the coating acts as a lubricant and protects the aluminum layer against scratching. A stylus electrode in contact with the graphite-containing surface coating is caused to move across the surface of the plate and is pulsed in accordance with incoming picture signals. The resultant current flow between the electrode and the thin metal coating is by design large enough to erode away the thin metal coating and the overlying conductive graphite containing surface coating, thereby exposing the underlying ink-receptive plastic substrate on the areas of the plate corresponding to the printed portions of the original document. This method of making lithographic plates is disadvantaged in that the described electroerosion process only works on plates whose conductive surface coatings are very thin; moreover, the stylus electrode which contacts the surface of the plate sometimes scratches the plate. This degrades the image being written onto the plate because the scratches constitute inadvertent or unwanted image areas on the plate which print unwanted marks on the copies.
Finally, we are aware of a press system which images a lithographic plate while the plate is actually mounted on the plate cylinder in the press. The cylindrical surface of the plate, treated to render it either oleophilic or hydrophilic, is written on by an ink jetter arranged to scan over the surface of the plate. The ink jetter is controlled so as to deposit on the plate surface a thermoplastic image-forming resin or material which has a desired affinity for the printing ink being used to print the copies. For example, the image-forming material may be attractive to the printing ink so that the ink adheres to the plate in the areas thereof where the image-forming material is present, and resistant to the "wash" used in the press to prevent inking of the background areas of the image on the plate.
While that prior system may be satisfactory for some applications, it is not always possible to provide thermoplastic image-forming material that is suitable for jetting and that also has the desired affinity for all of the inks commonly used for making lithographic copies. Also, ink jet printers are generally unable to produce small enough ink dots to allow the production of smooth continuous tones on the printed copies; in other words, the resolution is not high enough.
Our approach to lithographic printing involves applying images to a lithographic printing plate by altering the plate surface characteristics at selected points or areas of the plate using a non-contacting writing head, which scans over the surface of the plate and is controlled by incoming picture signals corresponding to the original document or picture being copied. The writing head utilizes a precisely positioned high-voltage spark discharge electrode to create on the surface of the plate an intense-heat spark zone, as well as a corona zone in a circular region surrounding the spark zone. In response to the incoming picture signals and ancillary data (such as dot size, screen angle, screen mesh, etc.) keyed in by the operator and merged with the picture signals, high-voltage pulses having precisely controlled voltage and current profiles are applied to the electrode to produce precisely positioned and defined spark/corona or plasma discharges to the plate which etch, erode or otherwise transform selected points or areas of the plate surface, rendering them either receptive or non-receptive to the printing ink that will be applied to the plate to make the printed copies.
Lithographic plates are made ink-receptive or oleophilic initially by providing them with surface areas consisting of unoxidized metals or plastic materials to which oil and rubber-based inks adhere readily. On the other hand, plates are made water-receptive or hydrophilic initially in any of three ways. One plate embodiment is provided with a plated metal surface, e.g. of chrome, whose topography or character is such that it is wetted by surface tension. A second plate has a surface consisting of a metal oxide, e.g. aluminum oxide, which hydrates with water. The third plate construction is provided with a polar plastic surface which is also roughened to render it hydrophilic.
Our apparatus can write images on all of these different lithographic plates, regardless of whether the surface is ink-receptive or water-receptive. In other words, if the plate surface is hydrophilic initially, our apparatus will write a positive or direct image on the plate by rendering oleophilic the points or areas of the plate corresponding to the printed portion of the original document. On the other hand, if the plate surface is oleophilic initially, the apparatus will apply a background or negative image to the plate surface by rendering hydrophilic or oleophobic the points or areas corresponding to the background or non-printed portion of the original document. Because most documents have less printed area than non-printed area, direct or positive writing is usually preferred in order to minimize the amount of plate surface area that must be written on or converted.
Our plate imaging apparatus is preferably implemented as a scanner or plotter comprising a writing head that consists of one or more spark- or plasma-discharge sources, each of which includes (or consists entirely of) an electrode. The source (or sources) is (or are) positioned over the working surface of the lithographic plate and moved relative to the plate so as to collectively scan the plate surface. Each source is controlled by an incoming stream of picture signals, which electronically represent an original document or picture. The signals can originate from any suitable source such as an optical scanner, a disk or tape reader, a computer, etc. These signals are formatted so that the apparatus's discharge source or sources write a positive or negative image onto the surface of the lithographic plate that corresponds to the original document.
If the lithographic plates being imaged by our apparatus are flat, then the discharge source or sources may be incorporated into a flat bed scanner or plotter. Usually, however, such plates are designed to be mounted to a plate cylinder. Accordingly, for most applications, the spark- or plasma-discharge writing head is incorporated into a so-called drum scanner or plotter, with the lithographrc plate being mounted to the cylindrical surface of the drum. Our printing apparatus can also be utilized in conjunction with a lithographic plate already mounted in a press to apply an image to that plate in situ. In this application, then, the plate cylinder itself constitutes the drum component of the scanner or plotter.
To achieve the requisite relative motion between the writing head and the cylindrical plate, the plate can be rotated about its axis and the head moved parallel to the rotation axis so that the plate is scanned circumferentially, with the image on the plate "growing" in the axial direction. Alternatively, the writing head can move parallel to the drum axis and the drum incremented angularly after each pass of the head, so that the image on the plate grows circumferentially. In both cases, after a complete scan by the head, an image corresponding to the original document or picture will have been applied to the surface of the printing plate.
As each discharge source traverses the plate, it is maintained at a very small fixed distance above the plate surface and cannot scratch that surface. In response to the incoming picture signals, which usually represent a halftone or screened image, each source is pulsed or not pulsed at selected points in the scan depending upon whether, according to the incoming data, the source is to write or not write at these locations. Each time the source is pulsed, a high-voltage spark or plasma discharge occurs between the electrode tip associated therewith and the particular point on the plate opposite the tip. The heat from that discharge and the accompanying corona field surrounding the spark etches or otherwise transforms the surface of the plate in a controllable fashion to produce an image-forming spot or dot on the plate surface. This dot is precisely defined in terms of shape and depth of penetration into the plate.
Preferably, the tip of each discharge-source electrode is pointed to obtain close control over the definition of the spot on the plate that is affected by the discharge from that source. Indeed, the pulse duration, current or voltage controlling the discharge may be varied to produce a variable dot on the plate. Also, the polarity of the voltage applied to the electrode may be made positive or negative depending upon the nature of the plate surface to be affected by the writing, that is, depending upon whether ions need to be pulled from or repelled to the surface of the plate at each image point in order to transform the surface at that point to distinguish it imagewise from the remainder of the plate surface (e.g., to render it oleophilic in the case of direct writing on a plate whose surface is hydrophilic). In this way, image spots that have diameters on the order of 0.005 inch all the way down to 0.0001 inch can be written onto the plate surface.
After a complete scan of the plate, then, our apparatus will have applied a complete screened image to the plate in the form of a multiplicity of surface spots or dots which differ in their affinity for ink from the portions of the plate surface not exposed to the discharges from the scanning electrode.
Maintaining the discharge sources at a precisely controlled, small spacing above the surface of the plate blank during scanning presents a number of engineering-design difficulties. The tracking system that guides the head must respond quickly to changing plate surface features so as not to reduce the writing speed of the pulsing electrode. Furthermore, accuracy cannot be compromised because excessive source-to-plate distances result in degraded image quality, while contact therebetween can cause physical damage both to the electrode and to the plate. Furthermore, the area proximate to the discharge source presents an environment that is both electrically noisy and permeated by airborne particles during imaging. Depending on its construction, the plate itself can interfere with electrical measurements by exhibiting various magnetic characteristics.
In addition to equipment for precisely tracking the discharge sources during imaging, we have found that some plate materials require means for removal of debris thereafter. Although it is possible to manually remove this debris by wiping the imaged plate with a soft cloth, such an effort is inconvenient and interferes with efficient operation of an on-press imaging system.